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//! HDF5 file signature (magic bytes) detection.
use crate::convert::TryToUsize;
use crate::error::FormatError;
use crate::source::{BytesSource, Source};
/// The 8-byte HDF5 magic signature.
pub const HDF5_SIGNATURE: [u8; 8] = [0x89, b'H', b'D', b'F', b'\r', b'\n', 0x1A, b'\n'];
/// Search a [`Source`] for the HDF5 signature, returning its byte offset.
///
/// The HDF5 spec allows the signature at offset 0, 512, 1024, 2048, 4096, …
/// (powers of two starting at 512, plus offset 0). Only the candidate 8-byte
/// windows are read, so this works against a lazy streaming source without
/// pulling the whole file.
pub fn find_signature_in<S: Source + ?Sized>(source: &S) -> Result<u64, FormatError> {
let len = source.len();
let mut sig = [0u8; 8];
// Offset 0.
if len >= 8 {
source.read_at(0, &mut sig)?;
if sig == HDF5_SIGNATURE {
return Ok(0);
}
}
// Powers of two starting at 512.
let mut offset = 512u64;
while offset + 8 <= len {
source.read_at(offset, &mut sig)?;
if sig == HDF5_SIGNATURE {
return Ok(offset);
}
// `len` is whatever the source reports, and `Source` is public, so a
// caller supplies it. A source that overstates its length past 2^63
// would otherwise wrap this doubling to zero and re-read offset zero
// forever in a release build (a debug build panics on the multiply).
// There is no candidate offset past 2^63, so stop: the search falls
// through to `SignatureNotFound`, which is the answer either way.
let Some(next) = offset.checked_mul(2) else {
break;
};
offset = next;
}
Err(FormatError::SignatureNotFound)
}
/// Search for the HDF5 signature in an in-memory buffer, returning its byte
/// offset. Thin wrapper over [`find_signature_in`] for the buffered reader path.
pub fn find_signature(data: &[u8]) -> Result<usize, FormatError> {
find_signature_in(&BytesSource::new(data)).and_then(|off| off.to_usize())
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn signature_at_offset_0() {
let mut data = vec![0u8; 64];
data[..8].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Ok(0));
}
#[test]
fn signature_at_offset_512() {
let mut data = vec![0u8; 1024];
data[512..520].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Ok(512));
}
#[test]
fn signature_at_offset_1024() {
let mut data = vec![0u8; 2048];
data[1024..1032].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Ok(1024));
}
#[test]
fn signature_at_offset_2048() {
let mut data = vec![0u8; 4096];
data[2048..2056].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Ok(2048));
}
#[test]
fn signature_not_found() {
let data = vec![0u8; 8192];
assert_eq!(find_signature(&data), Err(FormatError::SignatureNotFound));
}
#[test]
fn signature_not_found_empty() {
assert_eq!(find_signature(&[]), Err(FormatError::SignatureNotFound));
}
#[test]
fn signature_not_found_too_short() {
assert_eq!(
find_signature(&[0x89, b'H', b'D']),
Err(FormatError::SignatureNotFound)
);
}
#[test]
fn signature_at_non_power_of_two_not_found() {
// Signature at offset 100 should NOT be found
let mut data = vec![0u8; 1024];
data[100..108].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Err(FormatError::SignatureNotFound));
}
#[test]
fn signature_prefers_earliest() {
// Signature at both 0 and 512, should return 0
let mut data = vec![0u8; 1024];
data[..8].copy_from_slice(&HDF5_SIGNATURE);
data[512..520].copy_from_slice(&HDF5_SIGNATURE);
assert_eq!(find_signature(&data), Ok(0));
}
/// A source that reports a length it cannot serve is searched a bounded
/// number of times and then given up on.
///
/// `Source` is public, so `len` is a number a caller supplies rather than
/// one a file measured. The candidate offsets double, and doubling past
/// 2^63 used to wrap to zero: a release build then re-read offset zero
/// forever, and a debug build panicked on the multiply. The read counter is
/// what separates the fix from that — the search has to *stop*, not merely
/// return the same verdict, and a source that keeps answering cannot be
/// told from one that is being asked forever any other way.
#[test]
fn a_source_that_overstates_its_length_is_searched_a_bounded_number_of_times() {
use core::cell::Cell;
struct Overstating {
reads: Cell<u32>,
}
impl Source for Overstating {
fn len(&self) -> u64 {
u64::MAX
}
fn read_at(&self, _offset: u64, buf: &mut [u8]) -> Result<(), FormatError> {
let n = self.reads.get();
self.reads.set(n + 1);
// Fail rather than hang if the search does not terminate, so
// the defect this test covers surfaces as a failure.
if n > 1_000 {
return Err(FormatError::SignatureNotFound);
}
buf.fill(0);
Ok(())
}
}
let source = Overstating {
reads: Cell::new(0),
};
assert_eq!(
find_signature_in(&source),
Err(FormatError::SignatureNotFound)
);
// Offset 0 plus the 55 powers of two from 512 to 2^63.
assert_eq!(
source.reads.get(),
56,
"the signature search did not stop at the last representable offset"
);
}
#[cfg(feature = "std")]
#[test]
fn signature_found_over_a_streaming_source() {
use crate::source::ReadSeekSource;
// The signature at 512 is found by reading only the 8-byte candidate
// windows from a lazy Read+Seek source — never the whole buffer.
let mut data = vec![0u8; 1024];
data[512..520].copy_from_slice(&HDF5_SIGNATURE);
let src = ReadSeekSource::new(std::io::Cursor::new(data)).unwrap();
assert_eq!(find_signature_in(&src), Ok(512));
}
}